1. Field of the Invention
The present invention relates to an apparatus and a method to remove a resonance frequency of a system, and more particularly, to an apparatus and a method to remove a resonance frequency of a system in which an unnecessary resonance frequency, which otherwise occurs in a head stack assembly (HSA) having a head used to read/write data, is identified and filtered to prevent mechanical damage caused by resonance.
2. Description of the Related Art
In general, a hard disk drive (HDD) is a data storage system that includes a head disk assembly (HDA). The HDA includes instrumental parts, an electric circuit and a head stack assembly (HSA). The HSA generates a harmful resonance that is directly apparent in a position error signal (PES), and which deteriorates the stability of a servo tracking control of the HDD.
To solve this problem, a conventional method analyzes the resonance frequency from a frequency of the PES of the HSA. The analyzed resonance frequency is filtered or screened to remove the resonance frequency. The filters used to remove the analyzed resonance frequency were designed based upon resonance frequency components, which are identified in the PES of a predetermined number of HDDs, in the early development stage and applied to the HDDs. However, the resonance frequency changes according to characteristics of the parts making up the HSA and the characteristics of the assembly of the parts. The change in the resonance frequency between HSAs having such different characteristics cannot be solved using fixed filter coefficients applied to the HDDs.
A method of removing the resonance frequency of HSAs using an adaptive filter has been developed to solve this problem. This method includes identifying the resonance frequency in a frequency domain after a PES undergoes a frequency transformation process. The frequency transformation process includes processes such as a discrete fourier transformation (DFT), a fast fourier transformation (FFT), a wevelet transformation, and the like. The identified resonance frequency is filtered using a programmable filter.
Specifically, as shown in FIG. 1, an apparatus to remove the resonance frequency of the HDD includes an HDA 101, a read/write (RAN) channel processor 102, a controller 103, a resonance frequency identifier 104, a programmable filter 105, and a voice coil motor (VCM) driver 106. Here, the R/W channel processor 102 includes a pre-amplifier.
The R/W channel processor 102 amplifies signals read from a transducer of an HSA included in the HDA 101. Burst signals corresponding to servo information are separated from the amplified signals and operated by a servo control algorithm to generate a PES. If a command is input to carry out a process of identifying the resonance frequency of the HSA after completion of the whole process of manufacturing the HDD, the controller 103 applies a control signal CT to analyze the resonance frequency. The control signal CT is applied to the resonance frequency identifier 104. Also, the controller 103 generates a control output signal UK to drive a VCM based on a speed error signal and an acceleration error signal. The speed error signal and the acceleration error signal are analyzed by the PES and the servo control algorithm.
When the resonance frequency identifier 104 receives the control signal CT, the resonance frequency identifier 104 performs the frequency transformation to the PES and identifies the resonance frequency fr in a frequency domain. Once identified, the programmable filter 105 resets coefficients thereof, filters the control output signal UK, outputs the control output signal UK to the VCM driver 106, and drives the VCM in order to remove the identified resonance frequency fr.
However, the resonance frequency identifier 104 limits a frequency bandwidth which can be identified from the PES due to a servo sampling ratio of the HDD. In other words, the frequency identified by a process of frequency transformation to the PES is limited within the Nyquist frequency, which is half of a servo sampling frequency. If an aliasing phenomenon occurs due to a sampling effect and resonance frequency components fr1 and fr2 occur within a range between the Nyquist frequency and the sampling frequency, the resonance frequency components fr1 and fr2 are identified as resonance frequencies fr1′ and fr2′. The resonance frequencies fr1′ and fr2′ exist within the Nyquist frequency fn due to a mirroring phenomenon as shown in FIG. 7. Since the frequencies fr1′ and fr2′ are not the correct resonance frequencies, the programmable filter 105 cannot accurately remove the resonance occurring in the HSA since the programmable filter 105 is set to correspond to the misidentified resonance frequencies fr1′ and fr2′.